Calibrator for field intensity recorders



Nov. 29, 1949 c, v. LARRlcK 2,489,908

CALIBRA'I'OR FOR FIELD INTENSITY RECORDERS Filed 001.. 29, 1946 ChaFIeSVLaFPiGR,

by MW His Attorney Patented Nov. 29, 1949 UNITED STATES CALIBRATORZ FsfiR FIELD INIENgI-TY RECORDEBS Charles V. Larrick, Richland, Wasln, assignorsio General Electric Company, a-morporation of New York- Application October 29, 19m-s ea wmpass2 seams. (cum-e95) My invention relates to calibrating circuifiS and more particularly to a new and improved calibrating circuit for-afieldfintensity recorder.

The measurement of field intensities existing quencytransmitter"necessitates 'the use of compact, portable instnuments of high sensitivity. In obtaining good results, it is necessary to calibfate "the*"set up -in*isome fashion whereby the Lreadingsbbtained" may be interpreted properly 10 meritsf th'e measuring device.

'and fr'equent calibration checks must'bemade-to insure continued accuracy of results. Itisde- Tsirablethat 'this'g-may' be done) quickly and easily under actual operating conditions'when-the' "ap- The; features of= the invention which are believed to be nov el and patentable will be pointed "out in the 'olaims appended hereto. For a better understandingof the invention, reference is "paratus"is"ready-'-for *(186' at the'chosen location. 15 d nthe following description t the. accom- ,Devices 'r'ne'asufingfthe fieIdintensity' caused "by aradiating-source, such as a'transmitterwith an" associated antennay'employ' receiving" antenna which are usually"designedfor'best'operapariying -drawing in= -which the single figure -discloses==a' circuit: embodying the principles of my invention.

*Ref'erringtothedrawing, a dipole-antenna 1 ti on t frequencyxrat which t energy is .530 is shown connected to a receiver Z through-the being radiated by the transmitter. is therefore an element in thecircuit utilized in measuring the desired quantitiesf It is' known that an antenna possesses an electrical charac- An antenna series en si cr M n by -n a 1 a sm ssion line 5 whose characteristic impedance is ed rs enatedib sth ymb wen 01 m m 5. is connected across the output terminalsof ireteristic usually referredto as radiation resistance ceiver 2 p b ym h fqllgh an attenuator, fl.

. whichzis-measurabl in Q m Elms, dipole antenna ;.in free space has a -g-radiation resistance of 7,5 ,ohmsgbut this :value may: lieganywherebe- ":tween 6,0;and ,100 ohms-because of proximityeffectsaof the earths .sur-fiace or-qothenabsorbing s qobiects such as buildings. ,Because of this -chang,e, the calibration of =thez circuit for oneilo- 268331011101 itheqantenna istafiected andaerroneous ;r'eading ;may resultat other jlocations unless the Thus a signal received by antenna i will be amplified by receiver lend its strength is measurable in terms of the output as indicated hymeter 6.

Means are provided whereby the receiver and its associated" elements -=may-- be calibrated so that the indication of meter 6 may be immediately converted into terms, of wfield intensity at the anten aJ. vThese. meansinclude a source-of high circuit risvdesi'gned-to minimize zithegefiect. ,:EO1-35 frequency energy such as .a ccalibrating oscillator similar reasons, it is :desirable touallow the garrangements and values of the other circuit elements to remain thesame during calibration as during actual operation of the measuring device.

wor B-JF. signal i generator -8..'who.se output, may be fed into the antenna circuit through aHtIansmissiontline 9. which..ispreferably identical elec trically with transmission line 5 and thus .has a It is"-an object of I my invention -=to providel-- a qmcharacteristic.impedance. also equal to. Z0. flhe calibrati ng circuit "for a' field -intensity recorder whereby accurate "calibration may be accomplish'ed quickly and easily. I

E It is another object of myinvention to provide a new and improved field intensity-measuring '45 device whereby the effects ofchange in antenna radiation resistance are minimized.

It is a further objectiof my invention to provide a new and improved calibration circuit zoutputi-terminals of transmission line -9 are; corie nected across a meter -;I 0 and-across a potential divider circuit consisting of .the three 5 resistances I I, I2, and I3 in series. A resistor l4- m ay also be connected; if desired.- across the output terminals of transmission line 9 for purposes to be described. The voltage appearingacross resistor H wheri' the oscillator is in oper'ationis made available to the receiver input terminals; by conwhereby calibration of the total measuring de-" necting resistor H' across-the-input-termina1s-of 3 transmission line 5 through resistors I5 and I6. The resistors 3, I5, II, I6, and 4 in series constitute a potential divider circuit across antenna I one part of which, resistor II, is also a part of the potential divider circuit consisting of resistors I2, II, and I3 in series. Thus the voltages received by receiver 2 may originate either at antenna I or across resistor H. Further, the voltage across resistor II, measurable by meter Ill, which causes the same deflection of output meter 6 as does some unknown voltage on antenna I, in the absence of voltage acrossresistor II, is a measure of the unknown voltage. Preferably, meter I0 is of the thermocouple type whereby its indicator may be placed near the, I

output meter 6 which may be at some distance from the antenna and its circuit elements. Calibration is thereby facilitated since all meters are in the same general location.

containing resistance, inductance, or capacitance singly or in combination, may also be utilized.

If complex impedances are used it may be preferable that the phase or power factor angle of each impedance be the same thereby eliminating unequal variations therein with respect to frequency.

Although each of the various circuit elements may have various values, it is preferable that their values bear an essentially definite relationship with respect to each other. One such set of values may be derived in the following manner. It is apparent that the degree of coupling between the output of the calibrating oscillator 8 and the input to receiver 2 is principally controlled by the value of resistor II functioning as a portion of the voltage divider circuit consisting of resistors I2, I I, and I3 across the output of the oscillator 8. Since the usual oscillator output is high in comparison with normal required receiver input, resistor I I is made small. If a value for resistor II is chosen the remaining values may be calculated as follows. Assume resistor II to be a 5 ohm resistance. The following values are already known.

R=radiation resistance of a dipole antenna in in free space=75 ohms Zo=surge impedance of the transmission line Zo:95 ohms for a chosen type It is general practice to match a receiver or oscillator to the transmission line employed. Thus the impedance looking into the transmission line towards the receiver or oscillator is equal to the surge impedance of the transmission line.

Let

Vi=voltage induced on antenna I V5=voltage resulting across the input terminals of transmission line 5 Vi1=vo1tage across resistor II caused by oscillator output R1=antenna resistance plus resistance of resistors 3and4 R2=resistance of resistors II, I5, and II; in series which is essentially equal to the resistance of resistors I5 and I6 only since resistor II is of small value in comparison with resistors I5 and I5 in series Ra=resistance of resistors I2 and I3 in series It is desirable thatthe voltage V11 caused by oscillation output have the same effect on receiver 2 as an equal value of induced voltage V1 on antenna I when each appearsseparately.

For V1 alone Rgzo Y z-i' o z o l R+ z o 1 z+ 1 o+ 2 o For V11 alone R Z a l'i O RIZO 11 R RIZO l 2+ l 0+ 2 0 from which Y =& 11 2 But Ki i

should equal It is apparent that they may be made equal by allowing R1 to equal R2 wherein the expressions simplify to But for maximum power transfer from the antenna to the receiver it can be shown that Thus R1=l90 ohms and resistors 3 and 4 are each equal to 57.5 ohms. Also R2=190 ohms and resistors I5 and I6 are each equal to essentially ohms.

An additional advantage in placing resistors 3 and 4 in series with antenna I becomes evident when considering the effect upon the circuit of an antenna radiation resistance other than that obtaining in free space. Thus if the radiation resistance is 60 ohms instead of 75 ohms as previously considered, although is no longer equal to This relationship exists between the calibrating voltage and the induced voltage of the antenna because V11 is introduced into the antenna circuit instead of to the receiver directly. Thus no correction factors need be applied to the calibration if the input impedance of the transmission lines changes because of improper termination at the receiver end, since the ratio satisfies Si is independentioiism bfihmsta is a definite adv nt eince. e sater.,, cei racri e eby assured.

Resistors l2 and 13 have values chosen appro priately towallowimetereI'flstoaregiistcrr r n within the most accuratepprtion of itsnlhng e if,1a .vo ltrneter with a lfagn g sfrormzeroto th e volts and a resistance of' ()(iohmslisused whe calibrating the receiver. at, ,1o,000;mi ero volts mum transmission line iresistancegvalues of, resistors [2 and [3, as calculated in. the,fol; lowlneemanne y be p a,

lf vg is ,to,.be 10,000 microvolt ,.then Vi must be .040 volt since R1 and R2 are each 2 times zt.

Resistor II and that portion of the circuit towards the antenna and receiver presents a resistance value of or R3=196 ohm's and resistors I2 and I3 may each be 98 ohms in value.

It is evident that resistors H, l2, l3, l5, and it constitute an H-network resistance attenuator having impedance elements selected in the manner hereinbefore set forth to give a desired value of attenuation and. a desired impedance looking into its output terminals. Although other forms such as a T-network attenuator can be used, the H-network form is preferable in the circuit illustrated.

It is preferable to terminate the transmission line 9 in its surge impedance. This may be accomplished by utilizing resistor M of proper Value. Thus lifizoofi's'ddes from which R14=233 ohms as the value of resistor l4.

In calibrating the field intensity recorder it is only necessary to regulate the calibrating oscillator to the proper frequency and output voltage and record the readings of meter l and of the output meter 6 under conditions of essentially zero induced voltage on antenna I. Since by design a definite determinable ratio between the reading of meter l0 and V exists the output meter 6 may be calibrated in terms of V5.

In operation as a recorder, the induced voltage on antenna 1 bears a definite and determinable relationship to V5 and therefore to the reading of output meter 6 and may be measured in terms of the reading of oujgiut meter 6. The calibrating oscillator is inoperative but not disconnected from the recorder circuit while such readings are taken.

It may be preferable to utilize an antenna structure of such nature that dipoles of appropriate length may be used for each frequency at which measurements are taken. Substitution of a different set of dipoles does not affect the calibration of the equipment.

As will occur to those skilled in the art, various different arrangements and combinations of the principles described above may be employed without departing from the true spirit and scope of the invention and I therefore do not wish to limit my-inventionto tlie particular arrangement de-.

What- 1 01341111 as-new and desire to secureby Letters PatentoftheUnited States, is:

1; Electra-magnetic field intensitymeasuring apparatus comprisingan antenna, aradio-frequency voltage measuring device, -a-- first transmission lineconnected between said antenna and the input terminals of 'said measuring-- device,- arr attenuator having its output terminals connected to the antenna end of -said transmissionline, a radio-frequencysignal generator; a second transmission'line-connectdbetween the output terminals of said signal generator and the input terminals of said attenuator, and a meter connected to measure the amplitude of the input signal to said attenuator.

2. Apparatus as in claim 1 having at least one resistor connected in series with and adjacent to said antenna.

3. Electro-magnetic field intensity measuring apparatus comprising a dipole antenna, two impedance elements respectively connected in series with the two poles of said antenna, a radio receiver, a first transmission line connected between said two impedance elements and the input terminals of said receiver, indicating means responsive to the amplitude of the output signal of said receiver, an attenuator having its output terminals connected to the respective junctions of said two impedance elements and said first transmission line, a radio-frequency signal generator, a second transmission line connected between the output terminals of said signal generator and the input terminals of said attenuator, and a meter connected to measure the amplitude of the input signal to said attenuator.

4. Apparatus as in claim 3 in which the two impedance elements are equal resistors.

5. Apparatus as in claim 3 in which the impedance looking into the output terminals of the attenuator is substantially twice the characteristic impedance of the first transmission line.

6. Apparatus as in claim 3 in which the impedance looking into the output terminals of the attenuator is twice the characteristic impedance of the first transmission line and the impedance looking toward the antenna from the junctions of the two impedance elements and the first transmission line is substantially twice the characteristic impedance of the first transmission line.

7. Apparatus as in claim 3 having a resistor connected across the input terminals of the attenuator, said resistor having a value such that the second transmission line is terminated in its characteristic impedance.

8. Apparatus as in claim 3 in which the attenuator is an H-network resistance attenuator.

9. Electro-magnetic field intensity measuring apparatus comprising a dipole antenna, two equal resistors respectively connected in series with the two poles of said antenna, a radio receiver, a first transmission line connected between said two resistors and the input terminals of said receiver, said two resistors having such values that the impedance looking toward the antenna from the junctions of the two resistors and the first transmission line is substantially twice the characteristic impedance of said-first transmission line, indicating means responsive to the amplitude of the output signal of said receiver, an H-network resistance attenuator having its output terminals connected to the respective junctions of said two resistors and said first transmission line, the impedance looking into the output terminals of the attenuator being twice the;

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Bruce Nov. 11, 1930 Darboard Aug. 22, 1933 Bond Sept. 15, 1942 Paul June 29, 1943 Wang Apr. 16, 1946 George Apr. 30, 1946 Ramo Nov. 26, 1946 

